A good chunk of the value for traded in videogames surely must be put towards purchasing new games. So I don't think it's a given that new game sales will not only not decrease, but will instead grow. Such a decision isn't going to keep every single new purchase that would've otherwise been made in year's past while adding purchases from individual's that would've bought a used copy in year's past.
And a good number of people aren't buying games exclusively used or new but rather a combination of the two. Taking away the ability to buy used games will eliminate some of these customer's at least for a time until prices drop and some very well might stay away permanently.